In vehicles of this type there are today control units with a stored gear selector strategy, i.e. a time-based shifting sequence as a function of road incline, for example. A known technology is described in U.S. Pat. No. 5,832,400. For vehicles with a conventional automatic transmission, where the transmission shifts sequentially with a torque converter, there is a gear selection strategy based on an algorithm which takes into account a measuring point in the topology surrounding the vehicle, with instantaneous vehicle position as a reference point. By determining, by various methods, where the vehicle will be after a certain time interval, it is possible to modify the engine setting and the shifting points for the automatic transmission, i.e. at which rpm the transmission should shift up or down. Possible variants could be to use electronic maps together with a positioning system (e.g. a Global Positioning System, GPS) or extrapolate a future position for the vehicle. One disadvantage of this system is that it does not take into consideration how the road varies in elevation between two points of measurement, and extreme points (e.g. the crest of a hill) between the two points of measurement are thus not taken into account in certain cases. The engine and the transmission are set in accordance with the known technology, on the basis of how great the difference in elevation is between the two points of measurement, and the instantaneous throttle position. Throttle position means in this case and in the following text both an adjustable cruise control and an accelerator pedal.
U.S. Pat. No. 5,832,400 only takes into consideration, as was mentioned, a single point of measurement during a certain time or distance into the future, in order to see if the instantaneous engine torque will be sufficient, or if the engine and/or transmission needs to be reset. It is also described how a plurality of points of measurement can be used but in that case a mean value thereof is used, thus providing one value for the required driving force. With a transmission which is shifted sequentially and with the method just described, there is an uncertainty in the system which results in tangible consequences in the form of less than satisfactory cruise control function, uneven acceleration and unnecessarily large exhaust emissions.
Today, cruise controls function by controlling a throttle opening or a braking process, which is described in U.S. Pat. No. 5,894,731, until a selected or given target speed in reached. In practice this means that if a vehicle is moving downhill at a speed exceeding the target speed, the cruise control will attempt to brake the vehicle down to the target speed, regardless of the road incline after the end of the downhill stretch. This means that if, directly after the downhill stretch, there is an uphill incline, and the vehicle has been braked down to the target speed, the vehicle, if the uphill stretch is sufficiently steep, will move at below the target speed. The cruise control, in this situation, will compensate by greatly increasing the throttle opening and shifting down. Another example of the same type of problem is the case of the extension of the problem just described, where the vehicle is moving uphill at lower than target speed and reaches the crest, whereupon the continued open throttle will result in excess speed in the subsequent downhill stretch, and the vehicle will be forced to brake to once again reach the target speed.
There are today other types of cruise controls, for example those which use radar and adjust the vehicle speed to traffic in front, or transponders mounted on road signs for example. In both cases the target speed of the vehicle is changed continually depending on the surrounding traffic, weather conditions, wild animals other dynamic or random parameters. The purpose of the present invention is to achieve a motor vehicle of the type described by way of introduction, which removes the above mentioned problems by providing a system and a method, which, with the aid of a cruise control function, controls throttle opening and braking, where the braking is effected using auxiliary brakes for example, to obtain lower vehicle fuel consumption. There are also achieved lower noise and exhaust emissions, more even acceleration and more comfortable cruising.